The present invention relates to a process of forming a thin, heat-sealable layer on an oriented web.
The use of oriented films and fibers is widespread, particularly with semi-crystalline thermoplastic polymers. These oriented products are characterized by high tensile strength and high modulus. Orientation also influences crystalline order and hence the melting or softening point of the oriented polymer comprising the web. A problem with these oriented polymers is their poor heat sealability. Traditional oriented polymer films are heat sealed by the entire film being thermally welded above its melting temperature. This type of heat sealing by welding results in severe shrinkage and warping in the weld region. Welding also destroys the orientation in the weld area resulting in a weakened film or web in this critical area.
Efforts to make oriented semi-crystalline films heat sealable have generally focused on providing a thin layer of a heat sealable polymer on the film outer surface by coextrusion, sequential extrusion or coating. However these processes are problematic in terms of manufacturability of the film and adherence between the two layers. The heat sealable layer being formed of a lower molecular weight, lower softening point and/or less crystalline polymer also typically reduces the bulk tensile properties of the film. U.S. Pat. No. 4,247,591 proposes coextruding a thin outer layer of a higher average molecular weight version of the bulk polymer so both layers are subsequently oriented. The film is then selectively heated in these thin outer layers. The results reported allege an increase in joint strength of about 30 percent over single layer oriented films with a welded joint. Problems with this process include the need for careful selective heating to ensure that the heat seal remains primarily in the high molecular weight outer layers. The process also would not be suited to thin films of less than about 5 mils (127 .mu.m).
Also of concern with oriented films are the high gloss and low abrasion resistance of these films. A series of patents are directed at providing plastic strapping for industrial packaging use. The plastic strapping is formed from oriented polymeric films. The patents address the high gloss and abrasion resistance problems of these oriented films. U.S. Pat. Nos. 3,394,045 and 4,428,720 propose heating the oriented film to a fusion temperature (e.g., about 475.degree. F. (246.degree. C.) for polypropylene) by a flame or heated chromed roll. The depth of treatment is about 1 mil (25.4 .mu.m). The straps can then allegedly be used like conventional steel strapping to form crimped joints. However, this process is not well suited to providing heat sealable films. The high temperature and long dwell time of the treatment results in considerable oxidized species on the outer surface of the strapping, which is not a problem for crimped joints but which contaminates any heat seal joint. The high treatment temperature would also make manufacturability difficult for thin films due to the tendency of relatively thick melted polymer layers to adhere to rollers and associated machinery. U.S. Pat. No. 4,090,008 also reports that the above patents are deficient in preventing dusting or edge fibrillation and proposes a process for sealing only the edges with a heat treatment. U.S. Pat. No. 4,451,524 addresses the same fibrillation problem and purposes a subsequent orientation step following fusion of the surface layer (again about 1 mil (25.4 .mu.m)).
U.S. Pat. No. 4,822,451 summarizes various prior art film surface treatments and their effects, including; corona treatment, plasma treatment, sputter-etching, E-beam treatment, flame treatment, high intensity U.V. treatment and laser treatment. The object of the treatment process discussed in this patent is to treat a very thin surface layer of a film so that there is little or no chemical (except crystalline structure) or textural changes in the film polymer on the outer surface. All the summarized prior art treatments result in a change in the texture and/or chemical nature of the polymer surfaces being treated. The treatment described is a high intensity, high fluence laser (e.g., excimer laser) that when used with an appropriate semi-crystalline polymer creates a, preferably, 20-250 nm thick layer of treated polymer that is in a quasi-amorphous state. The quasi-amorphous state is characterized by loss of short range crystallization, but retains a limited level of long range orientation that can be seen if the treated layer is recrystallized. This treatment requires suitable matching of the laser wavelength to the absorption characteristics of the polymer and does not create any texturing of the treated polymer surface which is desirable in many applications thereby limiting its applicability.